Javamedium

`synchronized` vs `ReentrantLock`: what are the differences?

Answer

`synchronized` uses intrinsic monitor locks with automatic release. `ReentrantLock` is an explicit lock with features like `tryLock`, fairness policies, and interruptible lock acquisition—but you must release it in `finally`. Both are reentrant.

Advanced answer

Deep dive

Expanding on the short answer — what usually matters in practice:

Context (tags): java, concurrency, locks, synchronized
JVM: memory (heap/stack), GC, and what drives latency.
Contracts: equals/hashCode/toString, mutability and consequences.
Performance: boxing, allocations, collections, inlining.
Explain the "why", not just the "what" (intuition + consequences).
Trade-offs: what you gain/lose (time, memory, complexity, risk).
Edge cases: empty inputs, large inputs, invalid inputs, concurrency.

Examples

A tiny example (an explanation template):

// Example: discuss trade-offs for "`synchronized`-vs-`reentrantlock`:-what-are-the-"
function explain() {
  // Start from the core idea:
  // `synchronized` uses intrinsic monitor locks with automatic release. `ReentrantLock` is an 
}

Common pitfalls

Too generic: no concrete trade-offs or examples.
Mixing average-case and worst-case (e.g., complexity).
Ignoring constraints: memory, concurrency, network/disk costs.

Interview follow-ups

When would you choose an alternative and why?